Lets say there is a function

	bool halts(const char* program, const char* input);

which always returns true, if "program" when compiled with gcc, halts given the "input" on stdin.
and always returns false,  if "program" when compiled with gcc, runs forever.

Let's define:

    bool invert(bool x){
		if ( x ){
			for(;;){}
		}
		return true;
	}

Now, lets write in a file "test.c" source code for halts, and invert, and this main function:

	int main(int arg, char* argv[]){
		char* s = read_everything_from_stdin();
		invert(halts(s, s));
		return 0;
	}

Now lets run "gcc test.c -oa.exe && a.exe <test.c"

Now halts(s, s) evaluates if "test.c" with "<test.c" as input exits or runs forever.
If it returns "true", then invert(true) halts, so halt was wrong.
If it returns false, then invert(false) returns, so halt was wrong.